A superfamily of Calponin-Homology (CH) domain proteins is divided into two groups: structural proteins that organize the actin, microtubule, and intermediate filament cytoskeletons, and signaling proteins that control cytoskeleton assembly. We have described how the CH domain protein fimbrin crosslinks actin filaments into a bundle and surprisingly, that a fimbrin-containing bundle is anchored to the podosome in a macrophage cell. Furthermore, the podosome turns over its components rapidly, the lifetime is modulated by the microtubule cytoskeleton. We have discovered important parallels not only between podosomes and focal contacts but also with cell surface microvilli. This similarity suggests common rules for assembly of structure. In this renewal project we will identify and measure important structural, dynamic, and functional properties of the podosome with quantitative assays of mechanics and dynamics. This proposal has two specific aims: To describe how fimbrin bundles mechanically stabilize membranes and how tension affects assembly. Moving from structure and biochemistry to mechanics is the next step in understanding how the cytoskeleton carries out a structural role of controlling cell shape. To develop a model of podosome structure and assembly based on structural studies and quantitative assays of podosome dynamics and mechanics. In the background noise of actin assembly at the leading lamella, how do signals from the extracellular matrix and integrin receptors direct the assembly of a cell adhesion? Studies on CH domain proteins and cell adhesion are directly relevant to understanding underlying mechanisms of disease. The oncogenic properties of vav, the onset of myotonic dystony, blood disorders, and muscular dystrophy are caused by defects in different members of the OH domain superfamily.